1. Field of the Invention
The present invention relates generally to analog signal conditioning circuits and, more particularly, to conditioning circuits for removing noise components from an analog signal.
2. Description of the Prior Art
Copending U.S. patent application Ser. No. 796,621 (Anderson et al.), filed May 16, 1977, discloses a nephelometric system which measures the rate of change of a scattered light signal generated as a precipitate is formed during an antigen-antibody reaction. In such a system, a light beam is directed at a reaction zone and a detector monitors the light scattered by the precipitate. The precipitate is formed as the reaction proceeds, and the detected light scatter signal increases correspondingly in analog fashion with precipitate formation. It has been found that movement of certain particles in the reaction zone, such as air bubbles, dust, or even precipitate, will momentarily increase the amount of light scattered by the reaction. The momentary increases in light scatter add positive "unipolar" noise spikes on the fundamental analog scatter signal. Unless removed, the unipolar noise component of the analog output signal will introduce inaccuracies in the measurement.
In the past, efforts to remove unipolar noise components have employed high pass filters or the like to isolate the noise component of the analog signal. After isolation, the noise component is subtracted from the analog signal (i.e. is typically inverted and added to the signal to derive an output analog signal free to the noise component). Unfortunately, while such high pass filtering techniques can successfully isolate the high frequency term of the unipolar noise, they neglect the average level term of the unipolar noise. In this regard, such filters typically employ a coupling capacitor to extract the noise component. The capacitor charges to the average level of the noise component. The resulting noise signal component subtracted from the analog signal therefore excludes the average level term of the noise component so that a summing error is introduced in the output analog signal to the extent of the average level term.